The invention relates to an improved method and apparatus for drilling printed wiring boards with laser lights. More particularly, it relates to drilling blind via holes to connect between upper conductive layers and lower conductive layers.
FIG. 17 shows a diagram of a conventional optical system for drilling with laser light. In this laser drilling system, a laser beam 2 emitted from a laser head 1 is collimated and magnified or minified with a collimator 3, then shaped into a suitable diameter for drilling with an aperture 4. The shaped laser beam 2 is reflected by a corner mirror 5 and a mirror 14 in a machining head Z, then reflected by a pair of galvanometer mirrors 15a, 15b to a f-xcex8 lens 16. The laser beam 2 is positioned with the galvanometer mirrors 15a, 15b and incident vertically on a machining surface through the f-xcex8 lens. The machining is conducted on every machining area 18 defined by the size of f-xcex8 lens 16 and the area is moved by a X-Y table (not shown) from 181 to 18N sequentially.
FIG. 18a shows effects of the collimator 3 and aperture 4. The graphs on the lower part of this figure show distributional relationships between laser light energy (ordinate) and radial positions in laser beam (abscissa). Since the spatial energy distribution at the output window of the laser head 1 is a gaussian distribution in general, the spatial energy distribution of the laser beam passed through the collimator 3 is also a gaussian distribution. The size of the laser beam can be varied by magnifications (magnifying ratios or minifying ratios) of the collimator 3. That is, when the magnification is low, the diameter of the laser beam becomes small and the spatial energy distribution shows a high energy (or power) density profile of xe2x80x9caxe2x80x2-distribution (dotted line)xe2x80x9d shown in FIG. 18a, and when the magnification is high, the diameter of the laser beam becomes large and the spatial energy distribution shows a low energy (or power) density profile of xe2x80x9cbxe2x80x2-distribution (dotted line)xe2x80x9d shown in FIG. 18a. 
In particular, if the diameter of the aperture 4 is larger, the bottom of a machined via hole (i.e. the surface of an inner conductive layer) may be damaged because the energy is concentrated at the center. Therefore, an xe2x80x9cAxe2x80x2-distribution (solid line)xe2x80x9d or a xe2x80x9cBxe2x80x2-distribution (solid line)xe2x80x9d is made to avoid the damages by cutting out a central part of the beam, witch is a relatively homogeneous part of energy distribution, with the appropriate aperture 4. Hereafter, a full spatial energy distribution got by removing the aperture 4 from the optical path is called xe2x80x9cCxe2x80x2-distributionxe2x80x9d.
FIG. 18b, on the other hand, shows a spatial energy distribution when a beam homogenizer 30 is used in the optical path. The spatial energy distribution is shaped like a rectangle with the beam homogenizer 30, minified or magnified by a collimator 3 (xe2x80x9ca-distribution (dotted line)xe2x80x9d or xe2x80x9cb-distribution (dotted line)xe2x80x9d in FIG. 18b), then cut out with a aperture 4, made highly homogenized (xe2x80x9cA-distribution (solid line)xe2x80x9d or xe2x80x9cB-distribution (solid line)xe2x80x9d in FIG. 18b). Hereafter, these rectangle-like shaped distributions are called xe2x80x9ctop-hat shapedxe2x80x9d distributions and a full spatial energy distribution with the beam homogenizer 30, obtained by removing the aperture 4 from the optical path, is called xe2x80x9cC-distributionxe2x80x9d. Various commercial optical products can be used for the beam homogenizer 30, such as an aspheric lens system or a diffractive optical system.
Typical structures of printed wiring boards are an xe2x80x9cglass-containing substratexe2x80x9d (FR-4) which is a substrate laminated alternately with a layer or layers of conductor and a layer or layers of resin containing glass-fibers and whose surface layer is a conductive layer, an xe2x80x9cRCC substratexe2x80x9d which is a substrate laminated alternately with a layer or layers of conductor and a layer or layers of resin and whose surface layer is a conductive layer, and a xe2x80x9cresin-direct substratexe2x80x9d whose conductive layer is coated with a resin layer. Epoxy or polyimide is mainly used as the resin. Instead of glass-fibers, ceramic materials are sometimes used to reinforce the resin layer.
The following drilling methods with CO2 laser having a wavelength of 10.6 xcexcm are well known. A method of forming a blind via hole in the resin layer of a resin-direct substrate, called xe2x80x9cCO2 resin direct methodxe2x80x9d, was disclosed in xe2x80x9cGENERATING SMALL HOLES FOR IBMs NEW LSI PACKAGE DESIGNxe2x80x9d in IPC Technical Review, Pages 12-15, April 1982 and has been put to practical use. A method of forming a blind via hole in the resin layer of a glass-containing substrate with CO2 laser after forming a window previously by chemical etching or drilling was disclosed in Japanese Publication No. 58-64097 JP A1 and U.S. Pat. No. 5,010,232.
Moreover, a method for drilling through via holes or blind via holes in a substrate laminated alternately with multiple conductive layers and multiple resin layers was disclosed in Japanese Publication No. 01-266983 JP A1. That is a process of forming a window in a conductive layer by circular processing (in another word, xe2x80x9ctrepanningxe2x80x9d) with a ultraviolet (xe2x80x9cUVxe2x80x9d) laser light, which can effectively remove metals, and of drilling a resin layer with a CO2 laser light, repeatedly.
However, it is known that a thin residual resin layer (called xe2x80x9csmearxe2x80x9d) having thicknesses (tc) in the range of 0.2xcx9c3 xcexcm remains on the bottom of a via hole, in other words, just upon the conductive layer after the CO2 laser drilling. Furthermore, we found that the thickness of tc cannot be varied even if the energy densities or the numbers of shots of the CO2 laser pulses are variously changed.
The following is our speculation about a cause of the remaining. The CO2 laser drilling is a method utilizing thermal decomposition of resin layer at a temperature increased with the absorption of the infrared laser light. Hence, since the thermal conductivity of the (inner) conductive layer, for example, copper, is 1000 times higher than that of the resin layer, the thermal energy begins to flow into the inner conductive layer when the resin layer becomes thin. Therefore, the temperature of the resin layer cannot go up to the decomposition temperature in a thin residual layer and the residual layer of thickness 0.2xcx9c3 xcexcm remains consequently.
When the layer remains, a chemical desmear process is inevitable to remove the residual layer, which comprises steps for conditioning, washing, boiling, cooling, washing, swelling, washing, desmearing by oxidization, washing, neutralizing, washing, drying, etc. In this chemical desmear process, the wettability in the holes is low at diameters of the via holes less than 100 xcexcm, that is, it is difficult for the desmear fluid to enter the via holes deeply, and therefore the reliability of the process decreases. Moreover, there is a problem that diameters of the via holes by drilling with CO2 laser become 10 xcexcm larger in maximum usually, because the sides of the via holes are also cut 3xcx9c5 xcexcm by the desmear fluid though the purpose of the desmear process is to remove the residual layer on the bottoms.
On the other hand, A method of forming a blind via hole in the resin layer of a resin-direct substrate with a UV laser, called xe2x80x9cUV resin direct methodxe2x80x9d, was disclosed in xe2x80x9cExcimer Lasers: An emerging technology in the electronics industryxe2x80x9d in IPC Technical Review, Pages 16-20, November 1987 and has been put to practical use. A method of forming a via hole in a substrate laminated with conductive layers and resin layers only with a UV laser was disclosed in U.S. Pat. No. 5,593,606.
The residual layer does not exist on the bottoms of via holes with the UV laser, which is different from the CO2 laser method. However, if we use enough energy to obtain a practical processing speed, the surface of the conductive layer is to be also shaved by the excessive energy, and the surface roughness formed to plate firmly is melted and decomposed to a plain surface. In particular, when we use a wavelength-covert-type UV laser, whose wavelength is converted by a non-linear optical element and the like, it is easy to damage the surface of the bottom conductive layer because it is difficult to change the pulse energy during processing and the variation in the thicknesses of the resin layers is large about 20 xcexcm compared to the resin layer thickness of 65 xcexcm. Moreover, since the light energy reaching the bottom of the via hole is increased if the energy absorption coefficient of the resin layer is low, the stored light energy is increased just above the conductive layer. Thus, since a resin at a bottom of a via hole is decomposed and vaporized by the stored energy, and the resin layer at the bottom edge is peeled by the vapor energy. These damages can be avoided by decreasing the light pulse energy but the processing speed is decreased because the number of pulse shots must be increased.
When we process glass-containing substrates with a UV laser, the surface of the conductive layer is not only shaved by excessive energy, but the side wall of the via hole is also scooped like a barrel and the glass fibers protrude.
The following is our speculation about a cause of the excessive energy. The energy absorption coefficients at a wavelength of 355 nm in the UV light range are as follows: epoxy; 30-80%, copper; more than 70-75%, glass; about 20%. The thermal conductivity coefficients are as follows: epoxy; 0.8xcx9c0.85 Wmxe2x88x921Kxe2x88x921, copper; 386 Wmxe2x88x921Kxe2x88x921, glass; 1.04xcx9c1.09 Wmxe2x88x921Kxe2x88x921. These data show the very large differences between the materials. Therefore, since about 80% of applied laser energy is stored in the via hole by being reflected or diffused, the resin side wall of the via hole is scooped like a barrel and the glass fibers protrude, especially in pulse periods less then 3.3 ms (in pulse repetition rates higher than 3 kHz), and the reliability of the process decreases.
Moreover, since the laser processing energy is higher than 3 J/cm2 for drilling the conductive layer even of an RCC substrate, whose resin layer does not contain glass fibers, the difference between the material properties for the UV lights as mentioned earlier make it difficult to control the thermal conditions and the bottom surface of the conductive layer is damaged. Therefore, it is difficult to obtain the practical quality for the via holes.
In addition, since the energy spatial distribution of the UV laser beam is Axe2x80x2-distribution or Cxe2x80x2-distribution, as shown in FIG. 18a, a roughness on the bottom is created, and the time to remove the residual layer becomes longer or the bottom surface of the conductive layer is partially damaged.
A method for avoiding the damage to the bottom surface of the conductive layer is disclosed in, for example, xe2x80x9cLaser Ablation to sono-ouyoh,xe2x80x9d Corona Publishing co., ltd., 1999, P. 146, 11.6-13, which shows a selective resin layer etching method by setting the energy density of the processing laser light higher than the decomposition energy threshold of the resin layer and lower than the decomposition energy threshold of the conductive layer.
Here, the decomposition energy threshold is the energy density of the applied laser light that is necessary to start an ablation process, which is a decomposing, melting or vaporizing process with a laser light. The energy density of the applied laser light is a product of the applied power density and the pulse width (called a fluence).
On the other hand, a cleaning method for removing the residual materials or smears from the bottoms and the vicinity of via holes in a wide area was disclosed in Japan Pat. No. 2983481, by homogenizing an excimer laser beam widely in a line or a square distribution with a beam homogenizer device. However, when this method is applied to a resin-direct substrate, whose surface layer is a resin layer, the surface of the resin layer is damaged.
The first object of the present invention is, therefore, to provide a method for drilling printed wiring boards without a chemical desmear process by etching the residual layer selectively with laser light, which is made by measuring and utilizing the actual difference between the decomposition energy thresholds of resins and conductors.
The second object of the invention is to provide a method for drilling a substrate whose surface layer is resin, without surface damage.
The third object is to provide a suitable system to drill via holes and remove the residual layers with changing the energy densities of lasers and the wavelengths.
We found to achieve the first object that the resin layer contacting the targeted conductive layer is drilled to remain the residual layer as homogeneously as possible with a first UV laser beam whose energy density is higher than the decomposition energy threshold of the conductive layer and whose spatial energy distribution is top-hat shaped, and the residual layer is removed with a second UV laser beam whose energy density is lower than the decomposition energy threshold of the conductive layer and higher than that of the resin layer.
We empirically found the decomposition energy thresholds of materials at a wavelength of 355 nm are as follows: epoxy; 0.3xcx9c0.5 J/cm2, copper; 0.8xcx9c1.0 J/cm2, glass; 5xcx9c6 J/cm2. The present invention succeeded, with this small difference between the decomposition energy thresholds of the epoxy resin and the copper we found, to expose the targeted conductive layer by etching the residual layer selectively with a second UV laser beam hose energy density is lower than the decomposition energy threshold of the conductive layer and higher than that of the resin layer.
Moreover, the thickness of the residual layer should be homogenized with a top-hat-shaped first UV laser beam in this method. The present invention is the best to be utilized when a top-hat-shaped first UV laser processing and a second UV laser processing, whose energy density is lower than the decomposition energy threshold of the conductive layer and higher than that of the resin layer, are combined.
To achieve the second object, the spatial energy distribution of the second UV laser beam is made top-hat-shaped, and the diameter of the beam is made coincident with the diameter of the via hole formed in the resin layer with the first laser beam to avoid the damage to the resin layer surface.
The other method for the first object is forming the hole in the conductive layer with a first UV laser beam whose energy density is higher than the decomposition energy threshold of the conductive layer, processing the resin layer with a CO2 laser beam, and removing the smear with a second UV laser whose energy density is lower than the decomposition energy threshold of the conductive layer and higher than that of the resin layer, in sequence. In this embodiment, since a smear (a residual part of the resin layer) remains automatically with the CO2 laser, according to our experiment as mentioned earlier, it is easy to set the process parameters because it is needless to control the residual layer thickness. Moreover, the side wall of the resin layer is not scooped like a barrel as with the UV laser process.
To achieve the third object, at least two laser paths are prepared, the spatial energy distributions are made top-hat-shaped with beam homogenizer units in the paths, and the diameters and the energy densities are adjusted independently. If the paths are switched from a laser head with, for example, an acousto-optic deflector, it is good for saving space. Moreover, if the paths are aligned to a common axis near the surface of a substrate, the processing time is shortened because the table for the substrate is not necessary to move at the time of changing the paths.